<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div class="demo">
    <div class="box"></div>
</div>
</body>
<style>
    .demo {
        width: 300px;
        background: #60f;
    }
    .box {
        width: calc(100% / 2);
        background: #f60;
        height: 50px;
    }
</style>
</html>
